Exploring a more natural way to measure time
The 13-Month Calendar project exists to educate, inspire, and provide practical tools for understanding alternative timekeeping systems. We believe that how we measure time profoundly affects how we experience life, plan our activities, and connect with natural cycles.
Our goal is to make the International Fixed Calendar accessible to everyone through interactive visualization tools, educational content, and practical conversion utilities. Whether you're a calendar reform enthusiast, a history buff, or simply curious about alternative ways of organizing time, we're here to help you explore.
View any date in both Gregorian and 13-month formats with our side-by-side calendar display. See how dates map between the two systems.
Track moon phases alongside calendar dates. The 28-day month closely aligns with the lunar cycle, making moon tracking intuitive.
View sun angle, daylight changes, seasons, and Earth's orbital position. Understand how celestial events connect to our calendar.
Convert any date between Gregorian and 13-month calendars instantly. Find your birthday, anniversaries, or any important date.
The International Fixed Calendar (also known as the Cotsworth Plan or the Eastman Plan) was developed by Moses B. Cotsworth, a British accountant, in the early 20th century. The system divides the year into 13 months of exactly 28 days each, totaling 364 days. The 365th day is designated as "Year Day," a standalone holiday between years.
This calendar gained significant support from George Eastman, founder of Kodak, who implemented it for his company's internal operations from 1928 to 1989. The League of Nations seriously considered adopting it as a world standard in the 1930s, though religious objections ultimately prevented its global adoption.
We believe in making complex systems understandable and accessible to everyone.
Our tools are free to use and our approach is transparent and community-focused.
We encourage questioning established systems and exploring alternative possibilities.
Start Exploring
Ready to see the world through a different lens? Try our interactive calendar or convert your important dates to the 13-month system.
View Calendar Convert Dates